This invention relates to the removal of mineral or ash constituents from heavy hydrocarbon residues, and particularly from residues resulting from coal-oil coprocessing, residue hydrocracking and coal liquifaction.
Hydrogenation processes, such as hydrocracking, are commonly used for the conversion of heavy hydrocarbon oils to lighter products and for the coprocessing of heavy hydrocarbon oils and coal. When carbonaceous material, such as coal, is simultaneously hydrogenated with a heavy hydrocarbon oil, it undergoes liquifaction leaving behind particles consisting of carbonaceous material plus mineral material or ash which are inert to further hydrogenation. Thus, the mineral matter or ash (referred to hereinafter as "ash") becomes part of the heavy bottoms product or residue from the coprocessing.
The mineral matter or ash content of these residues can play a very important role in the economics of any processes for utilizing such residues. Because of the complex changes that the heavy hydrocarbon oils and mineral matter undergo, the reduction of ash particles from heavy hydrocarbon residues has proven to be a most difficult problem to solve.
In the past, a number of schemes have been tried for removing ash particles as part of coal liquefaction technology. Among techniques that have been attempted, there may be mentioned filtration, solvent extraction, anti-solvent deashing, and critical solvent deashing. In spite of extensive efforts to develop cost effective processes based on the above techniques, there still remains a need for a simple and inexpensive de-ashing process.
For instance, filtration of residues is most difficult to carry out because of the high viscosity of the mineral-containing hydrogenation residues. As a variation of this technique, filtration has been combined with centrifugation to accelerate the settling rate of the solids in the residues.
Solvent extraction of residues to separate ash is very simple in concept and works quite well on laboratory scale. However, in operations at commercial levels, the costs of solvents recovery become prohibitive.
In anti-solvent deashing, a so-called "antisolvent" is added to heavy hydrocarbon residues containing ash to dilute the residual oil and to promote the aggregation/coagulation of solids (mainly mineral matter) by the precipitation of preasphaltenes. Large agglomerates result and these settle at high rates. Subsequently, the residual oil is divided into two streams: an ash lean-stream and an ash-rich stream. Solids are removed by vacuum distillation of the ash-rich stream. It is also possible to use a centrifuge to further increase the particle settling rate.
In critical solvent deashing, an appropriate light hydrocarbon liquid and a super critical gas are used to solubilize ash-containing residual oil and to form low viscosity critical fluid. It has been known that a critical fluid solubilizes very large molecules. Ash particles settle rapidly by gravity in the critical fluid medium. Then, the critical fluid is divided into an ash-lean stream and an ash-rich stream. Ash is rejected from the ash-rich critical fluid stream by physical means, such as flashing, centrifugation or a combination of both. The clean residual oil is recovered by flashing the ash-lean critical fluid stream. The super critical gas and the light hydrocarbon liquid are recycled to the system. The phase behaviour of a multi-component critical fluid can be manipulated by adjusting temperature and pressure to cause phase separation within the critical fluid. When such process is applied to a decanter, a significant portion of the solids free critical fluid can be recovered without resorting to vaporization. This has been found to be a significant advantage over simple solvent extraction and it is known that the process works. However, the operation is sensitive to the nature of residues since the entire concept depends on the solubility of the residues to give a combination of super critical gas and light hydrocarbon. Moreover, the processing time is relatively long and the oil and solvent losses that leave with the rejected solids are high.
In all of the above processes, the ash particles settle through a viscous oil medium, which often requires dilution.
A de-ashing process is also described in Hardy, U.S. Pat. No. 2,789,083 in which a small amount of water is mixed with hydrocarbon oil and the mixture is allowed to settle to form a clear oil layer, a water phase and an aqueous emulsion layer. These layers are then separated and the emulsion is heated to a temperature above 500.degree. F. to break the emulsion as well as to decompose the oil soluble metallic compounds to metal fines which can then be removed by conventional means, such as filtration.
It is the object of the present invention to be able to remove ash particles from heavy hydrocarbon residues without the necessity of having the ash particles settle through the viscous oil.